The formation of electrophilic chlorine from carbon tetrachloride is catalyzed by liver microsomal cytochrome P-450. Stable isotope studies demonstrate that chloride ion is not the source of the electrophilic chlorine but rather the chlorine atom trapped is derived from CC14. These studies exclude a chloroperoxidase reaction as the mechanism of electrophilic chlorine formation. Molecular orbital calculations indicate the halogenating agent trichloromethyl hypochlorite (CC130C1). Experimental studies, however, indicate that the predominant mechanism involves the one electron reduction of CC14 to trichloromethyl radical (CC13) which reacts with 02 to form trichloromethyl peroxy radical (CC1300). Subsequent reactions of this intermediate leads to electrophilic chlorine. This reductive-oxygenation mechanism has important implications concerning the catalytic action of cytochromes P-450 and lipid peroxidation and toxicity produced by various xenobiotics.